2018 USA Umpire Exam

[Tru_in_Blu]

Next one up for discussion:

41) In a Fast Pitch game, the pitcher reaches down and places her fingers/hand in the dirt around the pitching plate and does not wipe the fingers/hand off before bringing them in contact with the ball. U3 calls an illegal pitch and explains to the pitcher and their coach that it is illegal to
place a foreign substance on the ball. The offensive coach protests that this is a misapplication of the rule as dirt is not a foreign substance.
What is the proper ruling?
a. Dirt is a foreign substance, so the illegal pitch is upheld.
b. The pitcher is warned if they apply a foreign substance to the ball again they will be ejected.
c. The illegal pitch call is reversed, as dirt is not considered a foreign substance.
d. Both a. and b. are correct.

I know the answer to this question, but cannot find the specific reference in the rules book. It was addressed in the March, 2010 Rules & Clarifications.

Any tips?